The present invention relates to an Automatic Gain Control (AGC) method, and more specifically, to an AGC method suitable for use in a Wireless Local Area Network (WLAN).
An automatic Gain Control (AGC) method is a process by which gain is automatically adjusted in a specified manner as a function of a specified parameter, such as received signal level. The gain is defined as the increase in the amplitude of an electrical signal from the original input to the amplified output. The following describes two exemplary AGC algorithms, binary tree search and RSSI-base, common AGC methods used in wireless communication receivers.
FIG. 1 illustrates the binary tree search AGC algorithm. The circuit is programmed to search the possible gain settings of Low Noise Amplifier (LNA) and Variable Gain Amplifier (VGA) in a binary fashion to quickly determine the final gain settings needed to optimize the inputs to the Analog/Digital (A/D) converters for demodulation. The LNA is for coarse gain adjustment, and the VGA is for fine gain adjustment. The binary tree search AGC algorithm is completely controlled by the detection of saturation of the A/D converter. The saturation of the A/D converter is determined by whether the digital word reaches a predetermined maximum/minimum value. The circuit implementing the binary tree search AGC algorithm initially sets the VGA and LAN pins to a predetermined maximum gain condition. Upon detection of A/D saturation, the circuit decreases the system gain via the VGA pin to a predetermined midpoint. The midpoint is chosen to allow the circuit to determine the correct setting of the LNA pin. If the circuit detects saturation at this midpoint, the circuit places the Radio Frequency (RF) front end to a low gain mode, and begins searching for the correct VGA setting in a binary tree fashion. If the circuit does not detect saturation on the A/D converters at this midpoint, the circuit leaves the LNA pin in the high gain mode and proceeds with the binary search of VGA. This binary tree representation of the AGC algorithm can also be illustrated by the AGC decision structure shown in FIG. 2. It is important to note that once the circuit makes a decision on the LNA setting, this LNA setting will remain for the entire duration of a packet transmission and will not be altered until the next packet. As shown in FIG. 2, the binary tree search AGC algorithm adjusts its gain by selecting one of two possible paths in every period. In order to achieve satisfactory gain accuracy, the required convergence time is relatively long.
FIG. 3 is a flowchart illustrating the RSSI-base AGC algorithm. A Received Signal Strength Indicator (RSSI) voltage is available as reference for gain adjustment in order to speed the gain control algorithm. The signal detected by the RSSI block is taken from the output of the base band analog IQ low-pass channel filters. The RSSI output level is directly dependent on the gain setting of the RF section. The gain setting of the base band AGC, however, has no impact on the RSSI output, since the input signal to the RSSI block is taken from before the base band AGC block. The RSSI-base AGC algorithm does not require analog to digital conversion since the gain can be fully controlled at the analog base band section of the receiver. The RSSI-base AGC algorithm, however, is only suitable for receivers comprising the RSSI function in the base band section. Unfortunately, not all RFIC venders provide RSSI function for base band transmission.